Yes the new (to me) grab offerings are a bit confusing but every time I've checked the just and car options are the same exact price. Sometimes taxi doesn't even appear and when it does it's WAY down so I have to scroll to even see it.
When I check grabtaxi to BKK (from ROS) it gives me a range (but have no way of knowing how accurate it is ) -

interesting the last time I took grab (taxi) from Lat Krabang to SGS the driver asked for 20 baht extra - but it hadn't shown the surcharge on the app - no big deal, but sometimes the app show NO extra charge and sometimes 25 or 45. Guess it's all about being an educated consumer.